1. Field of the Invention
The present invention relates to a lens drive system using a feed screw for moving one or more lens groups.
2. Description of the Related Art
An optical instrument such as a camera which is provided with a lens drive system using a feed screw for moving one or more lens groups is known in the art.
Such a lens drive system is provided with a feed screw shaft on which a male thread portion is formed, a nut member which is provided with a female thread portion which is engaged with the male thread portion of the feed screw shaft, a motor for rotating the feed screw shaft, and a lens frame or a lens holder for holding one or more lens elements. Rotating the feed screw shaft by the motor causes the lens frame to move together with the nut member.
In such a conventional lens drive system, if the feed screw shaft continues to be rotated by the motor until the lens frame or the nut member abuts against another member and stops thereby, the nut member and the feed screw shaft, which are engaged with each other, tightly fasten themselves on each other. This sometimes makes any further rotation of the feed screw shaft relative to the nut member impossible.
To prevent such a problem from occurring, it is generally the case that operation of the motor is controlled so that the motor stops rotating immediately after a position sensor senses that the lens frame has reached a predetermined stop position before the lens frame or the nut member runs over the stop position and abuts against another member.
However, there is a possibility that the position sensor cannot sense for some reason that the lens frame reaches a predetermined position. In this case, the feed screw shaft continues to be rotated by the motor until the lens frame or the nut member abuts against another member and stops thereby. As a result, the nut member and the feed screw shaft tightly fasten themselves on each other. This sometimes makes any further movement of the lens frame impossible.
Since one or more lens groups have been required to be moved by motor with a high degree of precision in recent years, there has been a tendency to reduce the pitch of the feed screw of the lens drive system. This tendency makes it easy to cause the nut member and the feed screw shaft to tightly fasten or lock themselves on each other.
The present invention provides a lens drive system which can reliably prevent the aforementioned problems which reside in conventional lens drive systems wherein the nut member and the feed screw shaft sometimes tightly fasten themselves on each other.
For example, a lens drive system is provided, including a lens frame having at least one lens element; a nut member which is secured to the lens frame, the lens frame and the nut member being movable in an optical axis direction without rotating relative to a base of the lens drive system, the direction of a screw hole of the nut member being parallel with the optical axis; a feed screw shaft having a male thread portion and a non-threaded portion, the male thread portion being engaged with a screw hole of the nut member so that the male thread portion is engaged with a female thread portion formed on an inner peripheral surface of the screw hole, a diameter of the non-threaded portion being smaller than an inner diameter of the screw hole; a motor for rotating the feed screw shaft; and a biasing member for biasing the nut member in a first direction from the non-threaded portion toward the male thread portion. When the feed screw shaft is rotated in a predetermined rotational direction by the motor to move the nut member and the lens frame in a second direction from the male thread portion toward the the non-threaded portion, the female thread portion is disengaged from the male thread portion to move onto the non-threaded portion before at least one of the lens frame and the nut member abuts against an element of the lens drive system other than the lens frame and the nut member and stops thereby.
It is desirable for the female thread portion to reengage with the male thread portion from the non-threaded portion by a rotation of the feed screw shaft in a rotational direction reverse to the predetermined rotational direction when the female thread portion is positioned on the non-threaded portion.
The biasing member can be a helical compression spring which is provided coaxially to the feed screw shaft.
The lens drive system can further include a pressing member having a through hole in winch the feed screw shaft is inserted. The pressing member is biased by the biasing member to press the nut member in the first direction from the non-threaded portion to the male thread portion.
It is desirable for a surface of the pressing member which presses the nut member to extend in a direction substantially orthogonal to an axis of the feed screw shaft.
It is desirable for a portion of the feed screw shaft to be fitted into the through hole of the pressing member with a minimum clearance between the through hole and the portion of the feed screw shaft when the female thread portion is positioned on the non-threaded portion.
The lens drive system can further including a second biasing member for biasing the lens frame in the first direction from the non-threaded portion toward the male thread portion. The nut member presses the lens frame to move the lens frame toward the non-threaded portion when the lens frame is driven to move in the second direction from the male thread portion toward the non-threaded portion. The lens frame moves toward the male thread portion by a biasing force of the second biasing member when the lens frame is driven to move in the first direction from the non-threaded portion toward the male thread portion.
The biasing member can bias the nut member in a first direction from the non-threaded portion toward the male thread portion when the female thread portion of the nut member is positioned on the non-threaded portion.
The biasing member can indirectly bias the nut member in the first direction from the non-threaded portion toward the male thread portion when the female thread portion of the nut member is positioned on the non-threaded portion.
It is desirable for the helical compression spring to be fitted on the feed screw shaft.
The lens drive system can be incorporated in a camera having a stop-position controller for optically detecting an axial position of the lens frame, for driving the motor in accordance with the detected axial position of the lens frame, and for stopping the lens frame at a predetermined axial position. If the stop-position controller malfunctions, the lens drive system operates to avoid an adverse effect on the lens drive system due to the malfunction.
Furthermore, if the stop-position controller malfunctions, the lens drive system operates so that the nut member and the feed screw shaft do not tightly fasten themselves on each other when the stop-position controller malfunctions.
It is desirable for the lens element, which is held by the lens frame, to be a focusing lens group of a photographing optical system.
The lens drive system can be incorporated in a zoom lens barrel. The lens element, which is held by the lens frame, can be a rearmost lens group serving as a focusing lens group of a photographing optical system. The base can be formed as a part of a housing of the zoom lens barrel.
It is desirable for the lens frame to be guided linearly in an optical axis direction without rotating about the optical axis.
According to another embodiment, a lens drive system is provided, including a lens frame having at least one lens element; a nut member which is secured to the lens frame, the lens frame and the nut member being movable in an optical axis direction without rotating relative to a base of the lens drive system, the direction of a screw hole of the nut member being parallel with the optical axis; a feed screw shaft having a male thread portion and a non-threaded portion, the male thread portion being engaged with a screw hole of the nut member so that the male thread portion is engaged with a female thread portion formed on an inner peripheral surface of the screw hole, a diameter of the non-threaded portion being smaller than an inner diameter of the screw hole; a motor for rotating the feed screw shaft; and a biasing member for biasing the lens frame in a first direction from the non-threaded portion toward the male thread portion. When the feed screw shaft is rotated in a predetermined rotational direction by the motor to move the lens frame in a second direction from the male thread portion toward the non-thread non-threaded portion, the female thread portion is disengaged from the male thread portion to move onto the non-threaded portion before at least one of the lens frame and the nut member abuts against an element of the lens drive system other than the lens frame and the nut member and stops thereby.
It is desirable for the biasing member to bias the nut member in the first direction from the non-threaded portion toward the male thread portion. The female thread portion returns onto the male thread portion from the non-threaded portion by a rotation of the feed screw shaft in a rotational direction reverse to the predetermined rotational direction when the female thread portion is positioned on the non-threaded portion.
It is desirable for the nut member to press the lens frame to move the lens frame toward the non-threaded portion when the lens frame is driven to move in the second direction from the male thread portion toward the non-threaded portion. The lens frame moves toward the male thread portion by a biasing force of the biasing member when the lens frame is driven to move in the first direction from the non-threaded portion toward the male thread portion.
It is desirable for the lens frame includes a pressing portion having a through hole in which the feed screw shaft is inserted. At least the pressing portion of the lens frame presses the nut member in the first direction from the non-threaded portion to the male thread portion via a biasing force of the biasing member.
It is desirable for a surface of the pressing portion which presses the nut member to extend in a direction substantially orthogonal to an axis of the feed screw shaft.
The lens drive system can be incorporated in a camera having a stop-position controller for optically detecting an axial position of the lens frame, for driving the motor in accordance with the detected axial position of the lens frame, and for stopping the lens frame at a predetermined axial position. If the stop-position controller malfunctions, the lens drive system operates to avoid an adverse effect on the lens drive system due to the malfunction.
Furthermore, if the stop-position controller malfunctions, the lens drive system operates so that the nut member and the feed screw shaft do not tightly fasten themselves on each other when the stop-position controller malfunctions.
The lens element, which is held by the lens frame, can be a focusing lens group of a photographing optical system.
The lens drive system can be incorporated in a zoom lens barrel. The lens element, which is held by the lens frame, can be a rearmost lens group serving as a focusing lens group of a photographing optical system. The base can be formed as a part of a housing of the zoom lens barrel.
It is desirable for the lens frame to be guided linearly in an optical axis direction without rotating about the optical axis.
In another embodiment, a lens barrel is provided, including a photographing optical system having of a plurality of lens groups, the plurality of lens groups including a focusing lens group, the lens barrel including a linear guide mechanism for linearly guiding the focusing lens group in an optical axis direction without making the focusing lens group rotate relative to a stationary barrel of the lens barrel; a nut member having a screw hole which is guided linearly in the optical axis direction without rotating relative to the stationary barrel via the linear guiding mechanism; a feed screw shaft having a male thread portion and a non-threaded portion, the feed screw shaft being inserted in the screw hole so that the male thread portion is engaged with a female thread portion formed on an inner peripheral surface of the screw hole, a diameter of the non-threaded portion being smaller than an inner diameter of the screw hole; a motor for rotating the feed screw shaft; a lens frame which holds at least one lens element and is movable together with the nut member; and a biasing member for biasing the nut member in a first direction from the non-threaded portion toward the male thread portion. When the feed screw shaft is rotated in a predetermined rotational direction by the motor to move the nut member and the lens frame in a second direction from the male thread portion toward the non-threaded portion, the female thread portion is disengaged from the male thread portion to move onto the non-threaded portion.
It is desirable for the linear guide mechanism to include at least one guide shaft which extends parallel to the optical axis to guide the lens frame linearly in the optical axis direction, the at least one guide shaft being engaged with the nut member to guide the nut member linearly in the optical axis direction.
The present disclosure relates to subject matter contained in Japanese Patent Application No.2001-394386 (filed on Dec. 26, 2001) which is expressly incorporated herein by reference in its entirety.